Extended Data Fig. 3: ICARUS model fit to light curve.

An example ICARUS toy model fitted to data between orbital phases 0.25 and 0.75 (ignoring data outside these phases), with the temperature of the dayside of the companion fixed to 10,462 K, the distance fixed to 1,140 pc, a fitted inclination of i ≈ 66° and an irradiated object with a radius of just 0.029R_⊙. The data from left to right illustrate the HiPERCAM u_s, g_s, r_s, i_s and z_s filters, with the dashed black lines illustrating the best-fit model in that filter. The light curves have been artificially shifted to line up with the u_s-band maximum light, as the ICARUS model is unable to capture the strong colour-dependent phase shifts seen in the data. We did not perform an MCMC over these models, as we found that there were acceptable fits for a wide range of inclinations and other parameters given the limitations of using a simple direct-heating model. Because we chose to fit the flux at peak, but not at minimum (where the wind contributes considerably), most models fit the peak flux well, but the excess flux at minimum increases gradually to the redder passbands, suggesting that there may be a colour dependence associated with the flux contribution from the wind, although this is difficult to disentangle from the flux contribution of the sdK at minimum light.